In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of oxides of nitrogen (NOx) and carbon monoxide (CO). Lower combustion emissions can often be attributed to a more efficient combustion process, with specific regard to fuel injector location and mixing effectiveness.
Early combustion systems utilized diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Diffusion type nozzles produce high emissions due to the fact that the fuel and air burn stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics.
An enhancement in combustion technology is the utilization of premixing, where the fuel and air mix prior to combustion to form a homogeneous mixture that burns at a lower temperature than a diffusion type flame and produces lower NOx emissions. Premixing can occur either internal to the fuel nozzle or external thereto, as long as it is upstream of the combustion zone. An example of a premixing combustor of the prior art is shown in FIG. 1. A combustor 8 has a plurality of fuel nozzles 18, each injecting fuel into a premix cavity 19 where fuel mixes with compressed air 6 from plenum 10 before entering combustion chamber 20. Premixing fuel and air together before combustion allows for the fuel and air to form a more homogeneous mixture, which will burn more completely, resulting in lower emissions. However, in this configuration the fuel is injected in relatively the same plane of the combustor, and prevents any possibility of improvement through altering the mixing length.
An alternate means of premixing and lower emissions can be achieved through multiple combustion stages, which allows for enhanced premixing as load increases. Referring now to FIG. 2, an example of a prior art multi-stage combustor is shown. A combustor 30 has a first combustion chamber 31 and a second combustion chamber 32 separated by a venturi 33, which has a narrow throat region 34. While combustion can occur in either first or second combustion chambers or both chambers, depending on load conditions, the lowest emissions levels occur when fuel, which is injected through nozzle regions 35, is completely mixed with compressed air in first combustion chamber 31 prior to combusting in the second combustion chamber 32. Therefore, this multi-stage combustor with a venturi is more effective at higher load conditions.
Gas turbine engines are required to operate at a variety of power settings. Where a gas turbine engine is coupled to drive a generator, required output of the engine is often measured according to the amount of load on the generator, or power that must be produced by the generator. A full load condition is the point where maximum output is drawn from the generator and therefore requires a maximum power from the engine to drive the generator. This is the most common operating point for land-based gas turbines used for generating electricity. However, often times electricity demands do not require the full capacity of the generator, and the operator desires for the engine to operate at a lower load setting, such that only the load demanded is being produced, thereby saving fuel and lowering operating costs. Combustion systems of the prior art have been known to become unstable at lower load settings, especially below 50% load, while also producing unacceptable levels of NOx and CO emissions. This is primarily due to the fact that most combustion systems are staged for most efficient operation at high load settings. The combination of potentially unstable combustion and higher emissions often times prevents engine operators from running engines at lower load settings, forcing the engines to either run at higher settings, thereby burning additional fuel, or shutting down, and thereby losing valuable revenue that could be generated from the part-load demand.
A problem with shutting down the engine is the additional cycles incurred by the engine hardware. A cycle is commonly defined as the engine passing through the normal operating envelope. That is, by shutting down an engine, the engine hardware accumulates additional cycles. Engine manufacturers typically rate hardware life in terms of operating hours or equivalent operating cycles. Therefore, incurring additional cycles can reduce hardware life and require premature repair or replacement at the engine operator's expense. What is needed is a system that can provide flame stability and low emissions benefits at a part load condition, as well as at a full load condition, such that an engine can be efficiently operated at lower load conditions, thereby eliminating the wasted fuel when high load operation is not demanded or incurring the additional cycles on the engine hardware when shutting down.